I've got a PIC18F26K22 connected to a MAX31855 thermocouple IC. The PIC is set to master SPI mode. To read the temperature and status etc. from the IC, you simply read 4 bytes.

If I scope the output of the serial data out of the chip with the wire disconnected from my PIC, the trace looks good, and the various bits appear correct (eg. If I short the thermocouple to ground or open circuit the thermocouple, bits 0 and 1 change from a 0 to a 1).

If I connect this wire to my PIC's serial data in line, the trace is erratic on the serial data line, and if I short or open the thermocouple, the bits that changed before do not change.

I'm sure the PIC is set up correctly as I have an LCD connected to the clock pin and the serial data out pin. What can be causing this problem I have.

Just a quick update. If I connect the serial out of the MAX31855 chip to a digital input of the PIC, the scope trace is as it should be. It's as if the SPI serial input on the PIC is messing up the data being transferred to it!